1. Field of the Invention
The present invention relates to a local area network communication systems suitable for use in combination of time-division multiplexing and frequency-division multiplexing in a multiple-channel broadband communications environment.
Many offices today use a network for transmission of data between personal computers, terminals and other devices within the office. Several types of architectures are used. In a star network, all nodes are coupled to a central point of the star, which provides centralized control of the flow of data. The central control on such a system can multiplex data in time from different nodes by alternately holding data in a buffer from one or the other transmitting node until a timeslot is available. A central control unit provides the synchronization necessary to insert data into assigned or available timeslots. One limitation of the star network is that the bandwidth available through the switch matrix is limited. Hence, means must be provided for multiple nodes to transmit data to the central control unit within a common broadband frequency channel.
A further system architecture employed for data communication is a token ring network. In a token ring network, a single cable or communication line is passed through each and every node, and a token packet is relayed from one node to the next, with the node desiring to transmit holding onto the token packet until its communication period is terminated. A node cannot transmit unless it has the token, and therefore, only one node is permitted to transmit at any time. This type of time-division multiplexing transmits data in irregular bursts, rather than in assigned time slots. This type of transmission is appropriate for data communications which typically occur in infrequent long bursts. Voice communications, on the other hand, require a substantially continuous connection over an extended period of time.
A still further data communication system architecture often employed for data communication is the Ethernet system. In the Ethernet system, a data node listens to see if the channel is in use before transmitting. While transmitting, the data node listens to determine if the data transmitted is received in the same form. If the received data differs, then it is assumed another node transmitted at the same time, resulting in a collision, and thus in scrambled data. The transmitting node then retransmits a packet at a random time later. Thus, neither central control of the network bandwidth acquisition of time slots nor central timing is needed.
2. Description of the Prior Art
The following patent was uncovered in a search for quadrature phase shift keyed (QPSK) demodulation systems as they might relate to the subject invention:
Rapid Acquisition Shift Keyed Signal Demodulator, U.S. Pat. No. 4,359,692. This patent describes a prior demodulator suitable for QPSK which requires a voltage controlled oscillator in the acquisition loop. The present invention does not require a voltage controlled oscillator in the acquisition loop and as a consequence is much faster in reaction.
The following patents were uncovered in reference to a Maximum Likelihood Detection Circuit as referred to in this application:
Carrier Synchronization and Symbol Synchronization in Offset QPSK Burst Communications, U.S. Pat. No. 4,313,205. This patent describes a carrier synchronization scheme based on average phase over a periodic interval and wherein phase synchronization is achieved by reliance on the filtered and demodulated upper sideband of the output of a delay and multiply circuit.
Method for Determining the Optimum Sampling Times for a QPSK or QAM Received Signal, U.S. Pat. No. 4,520,492. This patent describes a detector system wherein the amplitude of two quadrature components of a received signal are coupled and the point of identical amplitude is selected as the point of sampling.